The overall goal of this program project is elucidation of the fundamental mechanisms that govern the unique biology of hematopoietic stem cells. In the broadest sense, this program brings together laboratories expert in hematopoiesis and stem cell biology with those involved in studying pathways critical to vertebrate cell growth, proliferation, and differentiation. Recent studies of hematopoiesis have identified key contributions by several such pathways, including those activated by the Notch and Wnt receptor families which determine self-renewal and differentiation of hematopoietic precursors, and those of the Myc/Max/Mad network, acting downstream of the Wnt and Notch pathways, which function at the transcriptional level to regulate hematopoietic cell growth and proliferation. We propose to study the mechanisms by which these key pathways: 1) affect hematopoietic stem cell growth;2) are coordinated through interactions with each other;and 3) are impacted by the ubiquitin ligase Fbw7, whose function regulates the Notch and Myc pathways. Improved knowledge of these factors in the specific context of stem cell biology may provide opportunities to develop novel approaches for instructing the self-renewal and differentiation of hematopoietic and potentially other stem cell types, and may ultimately provide methods to generate appropriate cells for therapeutic purposes. Proposed projects include: Regulation of Hematopoietic Stem Cell Fate by Notch Ligands (Project 1);Functions of Myc and Mad Transcription Factors in Stem Cell Self-Renewal and Differentiation (Project 2);Coordinate Control of Hematopoietic Stem Cell Proliferation and Differentiation by the Fbw7 Ubiquitin Ligase (Project 3);and Combinatorial Signaling by Wnt and Notch Signaling Pathways (Project 4). Central to the success of this Program is the Cell Separation and Quantitative Transplantation Core which will provide FACS isolated stem cell populations, phenotypic analyses of cell populations undergoing differentiation in vivo or in vitro and quantitative transplantation assays to allow enumeration and functional assessment of manipulated stem cell populations. This core will enable investigators to tackle the critical but time-consuming and difficult task of assessing effects of the pathways under investigation. Administrative Core will facilitate overall interaction among investigators, and oversee integration of Core services with research projects.